Described herein is a method for detecting steady state ammonia slip for a motor vehicle having an internal combustion engine and an emissions control system. The emissions control system includes a selective catalytic reduction (SCR) device, a NOx sensor, and a controller. The controller executes a method for ammonia slip detection that includes determining if the SCR device is at steady state, comparing a NOx measurement from the NOx sensor with a predicted NOx value. If the NOx measurement exceeds the predicted NOx value by a threshold, perturbing a reductant injection, the perturbation having a selected magnitude and a selected duration. The method also includes measuring a NOx value resulting from the perturbation and computing a gradient thereof relative to the measured NOx, and ascertaining if a gradient of the NOx resulting from the perturbation exceeds a threshold and identifying a reductant slip condition if so.